Many medical devices utilize elongate flimsy members that are advantageously coated with various coating including biological coatings. Examples of one of these could be either guide wires or catheters. These items are often up to 100 inches in length and may have a diameter of less than 0.030 inches in diameter. A simple way to coat these flimsy elongate members is to dip them into a solution and then draw them out at a controlled rate. Often, especially for biological coatings, the coating material is so expensive and has such a limited pot life that mixing a large tank of material to dip the device into is cost prohibitive. The standard industry practice is to fill funnel tube with the coating material that is not much larger in diameter than the item to be coated to minimize the volume of fluid. A common funnel tube has an inner diameter of 0.375 inches. A “flimsy” elongate member can, for example, be defined as a member that cannot resist a force of 0.1 pound applied upwardly to the bottom of the member. The flimsy member will buckle rather than resist the force.
There are two basic problems that come up when you try to dip these flimsy elongate members into a funnel tube of coating material. The first problem is the initial insertion of the flimsy elongate member into the funnel tube. The surface tension and the viscosity of the fluid apply oppose the insertion force which can initiate a buckling of the flimsy elongate member. If the buckling is excessive the flexible elongate component will not be urged into the tube and/or the flexible elongate component could be damaged. It is very common for the coating materials to be very viscous(>50 centipose) and have a high surface tension. The most common way to get around this issue is to insert the flimsy elongate member into the funnel tube at a very slow rate (<2 inches/sec) that may allow some level of buckling while the tubing is being inserted. The problem with a very slow dipping rate is that it is slows down the production of these devices to such a slow rate that it is not practical to do this. The problem with allowing some buckling is that it is not very consistent from device to device making it very hard to automate and requiring human intervention.
When inserting these flimsy elongate members into the funnel tube filled with viscous fluid, it is common for the flimsy elongate member to come into contact with the sides of the funnel tube. When the inserted flimsy elongate member does contact the side of the tubing it'd resistance to being inserted into the funnel tube may increase making it more likely to buckle the flimsy elongate member.